1. Field of the Invention
The present invention relates to a hearing aid or acousticon coupled with a radio tuner. More particularly, it relates to a control system for a mixing circuit which mixes two audio signals, one of which is a hearing aid microphone signal continuously generated from the microphone of the hearing aid and the other of which is a radio signal generated from the radio tuner only when the radio is in operation.
2. Description of the Prior Art
A mixing circuit used in conventional audio devices was provided with an adjusting member for each audio signal for manually adjusting the level of the signal such that a desired ratio of the mixture (the ratio of the levels of the signals being mixed) is obtained by the adjustment of the manual members. When a hearing aid is coupled with a radio tuner to enable generation, through a single earphone, of a sound received by the microphone and amplified by the hearing aid and/or a sound produced by the radio tuner, it is necessary to mix the two audio signals such that the user can hear the sound from the microphone of the hearing aid when the radio is in operation because the sound from the microphone is always necessary for the user who is hard of hearing. To this end, one method may be to provide a level adjusting member for each audio signal and to mix the audio signals adjusted respectively as was done in the conventional mixing circuit. However, the provision of the adjusting devices increases the size of the hearing aid and makes the hearing aid cumbersome. Additionally, whenever the level of two audio signals changes e.g. due to an increase of voice level applied to the microphone, the mixture ratio changes and the adjusting device has to be used to adjust the ratio to a desired value. Thus, the above-mentioned hearing aid is inconvenient in requiring frequent adjustment.